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Plowing Bedrock: How bad is soil erosion in US cropland?

Plowing Bedrock: How bad is soil erosion in US cropland? thumbnail

A statement1 in JM Greer’s blog last month challenged everything I thought I knew about soil management in American cropland. At today’s rate of erosion, he wrote, the topsoil would be gone by 2075. Gone! The land might look like Providence Canyon7, where poor soil management in the 1820’s triggered runaway erosion that is still going on.

Providence Canyon, the “Little Grand Canyon” formed in southwest Georgia by manmade erosion in a tricky geological context. (permission requested)

“Gone by 2075” seemed an absurd projection for the well-tended midwestern topsoil. Driving there, you pass mile after mile of fields with furrows always running across the slope, patches of permanent grass where a gully might form, strips of grass and shrub along creeksides, sometimes bands of grass alternating with cultivated strips across steeply sloping fields. Many fields are no longer plowed or cultivated, but farmed on a no-till system.

You can’t stop all erosion, of course, unless in a rice paddy-type field. Most farmers seek a lesser goal4, formalized by the Soil Conservation Service in its early years: Keep erosion slow enough that it doesn’t damage the soil’s productivity.

How much erosion is acceptable? For the better soils of the Midwestern corn belt, about five tons8 per acre per year — in theory. The Soil Conservation Service, who are now the Natural Resources Conservation Service (NRCS), speak of the acceptable erosion rate as the Soil Loss Tolerance Rate, or sometimes the “T factor.”

NRCS reported in 20075 that only 28% of US cropland was eroding faster than the acceptable rate “T” — not an ideal situation, but apparently much better than “gone by 2075.” Yet the more detailed NRCS data don’t fully support this statement. And the T factor itself is a puzzling legacy.

NRCS publications don’t say how T was calculated, though it’s tabulated in the soil survey for every soil in every county of the US. The calculation took into account the natural rate of soil formation, but only as one factor among many. Judging from some discussions6 back in 1956, T was mainly designed to keep the topsoil deep enough for crop production, free of serious gullying, and capable of holding most of the nutrients applied to it. The soil formation rates considered may have been some early, highly optimistic estimates.

Current estimates suggest that soil forms at about half a ton per acre per year in some representative Iowa and Minnesota soils. Even the lowest T values are many times higher. Soil scientist Leonard Johnson6 reviewed the research behind the T factor in 1986. He concluded that erosion control based on T values “..should be considered as provisional or short-range.”

A 2014 webinar9 at ISU by Prof. Rick Cruse concluded that using T as the criterion for soil erosion control was effectively “a depletion schedule” for topsoil.

From R. Cruse, Pritchard Lecture 2014.10

The long range goal Johnson, Cruse and many others recommend is to keep topsoil erosion slower than the rate at which it is being replaced by nature. If this were achieved, hills and ridges would level out slowly over geologic time, always blanketed by a continually renewed layer of topsoil where crops could be grown.

Conservation planning is difficult because the accepted method of estimating soil erosion in a given field, refined and extended since the 1930s, is still inadequate. New studies, in which actual runoff9 from test fields is collected and measured, show that actual soil erosion is 100-200% worse than the most sophisticated estimate. (One of the test fields is being farmed by the best no-till methods.)

With actual soil erosion this severe, we may be losing topsoil at 10 to 30 times the rate it is forming. That range echoes the 10x-40x2 estimate of an Australian soil scientist8, for the entire world’s cropland. He told Time magazine in 2012 that

“A rough calculation of current rates of soil degradation suggests we have about 60 years of topsoil left. Even the well-maintained farming land in Europe, which may look idyllic, is being lost at unsustainable rates.”

Tolerable vs actual soil erosion rates in Europe. F.G.A. Verheijen, R.J.A. Jones, R.J. Rickson and C.J. Smith, Earth Science Reviews :24-38. From Prof. R. Cruse, Pritchard Lecture 201410

There are other soil management issues. No-till methods require herbicides, which have accumulated in groundwater to such an extent that many farm families rely on bottled drinking water. Herbicides can also kill grasses planted for erosion control.

Soil erosion estimates take into account a number of climate measurements, all of which may be changing as the earth warms. Revised values may be needed in the many tables of the soil survey. Some values reflect the frequency of heavy rainstorms, which may be increasing. A major storm can cause erosion close to the annual total in a single day.

Despite the questionable T-value guidance, NRCS remains an invaluable safety net, helping farmers to preserve and protect cropland. It assists in planning, financing and sustaining all types of erosion control structures. Among other services, it provides guidance for farmers making the transition from conventional to organic farming. All its work since 1933 has been helpful — just not as successful as we thought.

Holding erosion below the rate of soil formation isn’t impossible, but may require some unfamiliar and costly practices. Cover crops would help, as would the long-cycle crop rotations specified in every county’s soil survey. I’ll elaborate in another post, after I learn more.

Iowa cropland seen from the air after a rain. Light patches are subsoil exposed on hilltops and ridges. From Prof. R. Cruise, ISU webinar9.

Eclectications blog

10 Comments on "Plowing Bedrock: How bad is soil erosion in US cropland?"

  1. PrestonSturges on Mon, 29th Sep 2014 2:17 pm 

    This has always been the problem with row crops, but that’s because we rely on mechanical harvesters. Row crops became popular in part because of the labor shortages associated with harvesting tree crops, because nobody had figured out how to harvest things like pecans mechanically.

  2. Northwest Resident on Mon, 29th Sep 2014 2:43 pm 

    The sooner people get back to raising their own food on their own “mini-farms”, the better. Mechanized farming is destructive and unsustainable. In my raised planters, I experience zero topsoil loss. In fact, it is just the opposite, I am building my “topsoil” regularly with tried-and-true methods used through the centuries. It only takes 1/4 acre to provide ALL the food needed for a family of four, if done right, and still have plenty of extra left over for barter/trade/sale. And that 1/4 acre includes being able to grow enough grain and/or other feed for a bunch of chickens, meat rabbits, maybe a goat or cow. Read all about it: “Mini-Farming: Self Sufficiency on 1/4 Acre”. ht tp://

  3. J-Gav on Mon, 29th Sep 2014 5:06 pm 

    Soil and water are keys to any type of human survival on this globe.

    Sounds like you’ve got it workin’ for ya pretty well, NW. Even after reading Markham a while back, I always thought 1/2 acre would be nice for some fruit trees and other biomass (trees, hedges …). Davy and some others on this board have got quite a few acres so I guess they’re not hurtin’ for things to compost, burn, etc.

  4. Makati1 on Mon, 29th Sep 2014 8:31 pm 

    Topsoil thickness in the US Mid-west was supposedly over 6 feet when the pioneers migrated west. It is now less than 1/3 that depth due to farming and is melting away fast. If you can call it topsoil. more like dead sand and chemicals.

    I once read that it takes about 100 years for nature to build an inch of topsoil. Sounds about right. We can speed it up in our gardens with a lot of labor, but not on our farms. Too large. That topsoil is lost for the next few hundred/thousand years.

    Interesting that they mention rice paddys as a good example of soil retention. I see them everywhere here. Kinda labor intensive. But then, most people here still farm at some level or another. There are still more farmers here than city dwellers. Most will just return to their family’s farm when the SHTF. Thousands of unused acres here for them to spread out on.

  5. DMyers on Mon, 29th Sep 2014 8:51 pm 

    I’ve read that it was soil depletion that took down the Sumerian civilization, the very first, and the template of all to follow. What a huge statement, that we now face global soil depletion, as civilization serves to deplete and erode at whatever scale it encompasses.

    What a blindside this will be. Soil is ubiquitous. Everywhere you look there is soil, except at the mall. We couldn’t possibly run out of that, it would seem to any normal cretin.

    It’s always possible that a new “virtual soil” technology will emerge, launching us into a new dimension without limits. We might even engage a large number of large butted people to be propped up in fields across America to grow food plants from their anal cavities (organic at its best).

    Creativity is the key to surviving a world without soil. Think fast! Sixty years goes by very quickly.

  6. synapsid on Mon, 29th Sep 2014 9:21 pm 


    Part of what hit Sumer was salt buildup. We see this in records of harvests over decades and centuries, as the percentage of wheat declined and that of barley, more salt-tolerant, increased.

    A semi-arid climate, where irrigation is needed in order to grow crops for production not just family consumption, is a bad setting for irrigation. Put water on the land so it will soak in to reach roots and the water will dissolve minerals on the way down; after the irrigation, water will evaporate at the surface and some of the infiltrated water will move up to replace it carrying the dissolved material with it, and those dissolved substances will be left in the soil after that water evaporates away in its turn.

    This same thing has destroyed the fertility of much of California’s Imperial Valley, and it’s been an ongoing problem over much of the world for millennia.

  7. farmboy on Mon, 29th Sep 2014 9:51 pm 

    This article is only a small window into the big picture and also includes a myth or two. For some more substantial info you can check out these links and

  8. Norm on Mon, 29th Sep 2014 10:56 pm 

    This problem would be solved instantly, IF you dumped the Schmitt from the sewers onto the farmland. But nobody wants to do that. Its politically incorrect.

  9. Bogdan on Tue, 30th Sep 2014 2:39 am 


    Isn’t the “Schmitt from the sewers” mixed with a bunch of detergent, soap, shampoo and other stuff? Isn’t that stuff toxic for agriculture? I don’t know for sure, just asking. Maybe we need 2 separate sewers.

  10. Norm on Tue, 30th Sep 2014 7:44 am 

    Use the pipes that comes from Congress. Their B.S. is 100% pure.

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